Hailed by the Bulletin of the American Mathematical Society as "easy to use and a pleasure to read," this research monograph is recommended for students and professionals interested in model theory and definability theory. The sole prerequisite is a familiarity with the basics of logic, model theory, and set theory. The author, Professor of Mathematics at UCLA and Emeritus Professor of Mathematics,University of Athens, Greece, begins with a focus on the theory of inductive and hyperelementary sets. Subsequent chapters advance to acceptable structures and countable acceptable structures, concluding with the main result of the Barwise-Gandy-Moschovakis theory, which is the key to many applications of abstract recursion theory. Exercises at the end of each chapter form an integral part of the text, offering examples useful to the development of the general theory and outlining the theory's extensions.